The adaptation required of the left ventricle to meet the blood flow demands of the body when the heart is subjected to various stresses will be studied in conscious dogs previously instrumented with flowmeters for measuring the cardiac output, pressure cells for measuring the left ventricular pressure and dimension gauges for measuring the left ventricular internal diameter. The physiological stresses will include alterations in heart rate, filling pressure and arterial pressure. The pathological stresses will include hypertension and myocardial ischemia or infarction. The interrelationships of the three physical variables - flow, pressure and internal diameter will be fundamental to the analysis of the response of the left ventricle to these stresses. The left ventricle will be analyzed both as a pump and as a muscle organ. In addition to establishing the mechanical adaptation of the heart to stress, a major objective is to identify the various neural reflexes which contribute to the overall cardiovascular response. The role of the cardiopulmonary and arterial baroreceptor in the fine control of the cardiovascular system will be of major concern.